This invention constitutes an improvement on the kind of valve shown in U.S. Pat. No. 4,129,144, dated Dec. 12, 1978 and other like valves having the equivalent of the head or poppet shown at 30 therein. It has been found that at times the pipe end that is set into the chamber that the head is in will be allowed to closely engage the head and in effect close the valve altogether by mistake. This invention provides a simple inexpensive structure that permanently prevents such malfunction. No part of the valve need be changed, only the head referred to, which in this case is shown domed.